I can't give you the exact solution.
Exhaust always the deserted art in the automobile industries. Often people think exhaust is another piece of metal, but you know, the european spending millions to ensure the sound of exhaust in a bmw or mb or jaguar is tune to perfection.
Take example of ANSA who spended millions on a "sound room" to tune their exhaust, so when the vehicle or even truck are running of the road, the sound is tune like music to your ears.
The simple way you can do is to have a look and measure the diameter of the sample, and go ahead with "trial n error" error method will lead you to there....
It is a common practice the diameter of the exhaust system to start from smaller to bigger , this is because we need to create a vacuum situation where the exhaust gas will flow out from your engine to the environment safely. Or else you will be punishing your exhaust system to overwork.
So i see no harm for you to use a 2-1/4 dia rear exhaust.